The present invention relates to percussive extension drilling and, in particular, to a drill string arrangement for use in such drilling.
Extension drilling typically involves the use of a drill bit mounted at the end of a drill string which is both rotated and subjected to longitudinal impacts. The upper end of the drill string is connected to an above-ground drilling machine which performs the rotation and imparts the impacts. Such a percussive drilling technique is commonly referred to as bench drilling.
The drill string includes an adapter connected to and extending from the drilling machine, and at least one rod 10 (but usually a series of rods) connecting the adapter to the drill bit. In a rod drilling apparatus 8, shown in FIG. 1, each rod 10 has a male screw thread 12 at one end and a female screw thread 14 at the other end. The uppermost rod 10' has its female thread 14 connected to a male screw thread 16 of the adapter 18. The remaining rods 10 are joined together in series. The lowermost rod 10" has its male thread attached to a female screw thread of a drill bit 20. The adapter 18 and the rods 10 have respective central passages extending therethrough and aligned with one another for conducting flushing fluid (usually air) which exits through outlets formed in a front face of the drill bit to cool the cutters and flush-away cuttings. The cuttings, along with the flushing fluid, are discharged upwardly through a gap formed between the drill string and the wall of the hole being drilled.
A problem encountered in connection with the above type of drilling apparatus involves a tendency for the rods to bend during drilling, thereby reducing the drilling speed and drilling accuracy, as well as increasing the risk of the bit becoming stuck in the ground. To deal with that problem, it has been known to interconnect the drill bit with the lowermost rod by means of a tube 20 having a larger outer diameter than the rods (see FIG. 2). The tube, being stiffer than the rods, is better able to resist bending, so that the drill bit travels faster and straighter.
Notwithstanding the success of that drilling apparatus, room for improvement remains.
It is an object of the present invention to improve the performance of the above-described drilling apparatus especially as regards the efficiency of energy transfer and the speed of flushing fluid.